(2) A tissue is an aggregate of cells in an organism that have similar structure and function

The fundamental types of tissues in animals are epithelial, nerve, connective, muscle, and vascular tissues whereas in plants, they are the meristematic (apical meristem and cambium), protective (epidermis and cork), fundamental (parenchyma, collenchyma and sclerenchyma) and vascular (xylem and phloem) tissues. Tissues that work in unison to carry out a specific set of functions form an organ.

brown adipose tissue - a thermogenic type of adipose tissue containing a dark pigment, and arising during embryonic life in certain specific areas in many mammals, including humans; it is prominent in the newborn.

cancellous tissue - the spongy tissue of bone.

cartilaginous tissue - the substance of cartilage.

chromaffin tissue - a tissue composed largely of chromaffin cells, well supplied with nerves and vessels; it occurs in the adrenal medulla and also forms the paraganglia of the body.

nerve tissue - nervous tissue the specialized tissue making up the central and peripheral nervous systems, consisting of neurons with their processes, other specialized or supporting cells, and extracellular material.

tissue density
the penetrability of tissue by x-rays, bone and tooth being most dense, blood and soft tissue the next, fat the next, and gas and air least.

tissue edema
an abnormal accumulation of tissue fluid.

tissue factor
see tissue thromboplastin.

tissue fluid
the extracellular fluid that constitutes the environment of the body cells. It is low in protein, is formed by filtration through the capillaries, and the excess drains away as lymph. See also interstitial fluid.

tissue reacting agent
substances that have a poorly defined but advantageous local effect on tissues.

tissue receptor site
a cell receptor common to cells of a particular tissue.

tissue residue
residues of chemical substances that are unacceptable to local pure food legislation especially sulfonamides, estrogens, chlorinated hydrocarbons, heavy metals. These are thought or known to have a deleterious effect on people eating or drinking the relevant animal product. See also chemical food residue.

tissue sensitivity
the susceptibility of individual tissues to injury by x-ray. The injury may be by way of inflammation, necrosis or cessation of cell growth. Fast-growing tissues in which the cells have a high mitotic index are the most sensitive, especially gonads, germinative layer of skin and erythropoietic tissues.

supportive tissues
cartilage and bone.

tissue therapy
see glandular therapy.

tissue typing
identification of tissue types for purposes of predicting acceptance or rejection of grafts and organ transplants. The process and purposes of tissue typing are essentially the same as for blood typing. The major difference lies in the kinds of antigens being evaluated. White blood cells, particularly lymphocytes, are used for tissue typing. The acceptance of allografts depends particularly on the matching of MHC antigens. If the donor and recipient are not MHC identical, the allograft is rejected. See also typing.